Discharge lamps having improved thermionic cathodes

ABSTRACT

A thermionic cathode for discharge lamps is a two part structure consisting of a wire stem bearing a cup at its end, the cup being a receptacle for an electron-emitting composition and ensuring that electrons can only issue from the mouth of the cup and not by passage through the side of the cup. In one form, a foil is rolled around the end region of the stem, the rolled foil projecting beyond the end of the stem and thereby forming the cup. Instead of foil, a welded-coil may be used in a similar way to define the cup.

United States Patent Clarke et a1.

[4 1 Oct. 24, 1972 [54] DISCHARGE LAMPS HAVING IMPROVED THERMIONICCATHODES [72] Inventors: Maurice George Clarke; Norman 21 Appl. No.:113,934

[30] Foreign Application Priority Data Feb. 11, 1970 Great Britain..6,662/70 [52] US. Cl ..313/346 R, 313/184, 313/356 [51] Int. Cl...H01j l/l4, H01 j 19/06 [58] Field of Search ..313/356, 346, 184

[56] References Cited UNITED STATES PATENTS 2,175,345 10/1934 Gaides eta1 .4313/346 DC 3,548,242 12/1970 vAyotte et a1. ..313/l84 2,900,5548/1959 Woehling et a1 ..313/356 X 1,953,625 4/1934 Miesse ..313/356 XFOREIGN PATENTS OR APPLICATIONS 1,137,124 12/1968 Great Britain..3l3/346 OTHER PUBLICATIONS Albert et aL; New Type of CompositeAll-Metal Electron Emitter..." British J. Appl. Phys., April 1967, Vol.18, Pp. 627- 633.

Primary ExaminerDavid Schonberg Assistant Examiner-Toby l-l. KusmerAttorney-James Theodosopoulos [57] ABSTRACT A thermionic cathode fordischarge lamps is a two part structure consisting of a wire stembearing a cup at its end, the cup being a receptacle for anelectronemitting composition and ensuring that electrons can only issuefrom the mouth of the cup and not by passage through the side of thecup. In one form, a foil is rolled around the end region of the stem,the rolled foil projecting beyond the end of the stem and therebyforming the cup. Instead of foil, a welded-coil may be used in a similarway to define the cup.

3 Claims, 3 Drawing Figures PATENTED 3. 700,951

RICE GEORGE CLARKE N eess v ORS BY T ATTORNEY DISCHARGE LAIVIPS HAVINGIMPROVED THERMIONIC CATHODES The present invention relates to dischargelamps having improved thermionic cathodes.

According to the present invention there is provided a discharge lampincluding a thermionic cathode which comprises a refractory metal sternand a refractory metal tubular member which is secured to one end of thestem, the tubular member extending beyond the end of the stem to form acup and an electron-emitter composition being contained in the cup. Thesaid composition may include a refractory metal, a lanthanide metal orthorium and a further metal which is at least partially soluble in thelanthanide or thorium and whose alloy therewith is capable of wettingthe refractory metal in the said composition.

The particular metallic elements used can be any of those described inBritish Patent Specification No. 1,137,124 and in a paper published inthe British Journal of Applied Physics, 1967, 18, 627. Thus therefractory metal forming the stem and sleeve and the refractory metal ofthe said composition may be tungsten, tantalum or molybdenum, althoughtungsten is preferred.

The said further metal should have a low vapor pressure at the operatingtemperature of the lamp, which typically may be in the range l,400 to1,800C. The further metal can be an element in the first transitionseries, that is in the fourth period of the periodic classification ofelements, and may be any of the elements having atomic numbers 22 to 28.Alternatively it can be a precious metal selected from the transitionseries of the fifth and sixth periods. Suitable elements are thosehaving atomic numbers 44 to 46 or 76 to 78. Moreover, instead of being aGroup VIII element, the further metal could be a lanthanide if thecomposition includes thorium; if the composition includes a lanthanideinstead of thorium, then the further metal could be another metal in thelanthanide series. Optimum results are however obtained if the furthermetal is a Group IVa metal, that is titanium, zirconium or hafnium.

The said electron-emitter composition should contain 85 to 95 percent ofrefractory metal, up to several percent of further metal with thebalance being the said lanthanide or thorium. A satisfactory compositionis 90% W, 9.5%Th and 0.5%Zr. Other compositions which may be used aredescribed in the two abovementioned references.

The. said electron-emitting composition can be introduced by pressingdry powder into the cup formed by the end of the stem and the sleeve,and the cup need not be completely filled therewith. It may besufficient to provide the inner surfaces of the sleeve with a coating ofthe said composition. After applying the coating, the sleeve is heatedto sinter the said composition coating thereto.

In one embodiment, the tubular member is a refractory metal foil whichis wound or wrapped around the stem adjacent one of its ends. Thetubular member thus forms a sleeve around the stem and projects from theend thereof to form a hollow cylindrical cup. The foil should be wrappedupon itself so that in operation, emitted electrons and any evaporatedmetal atoms can only pass through the mouth of the cup and cannot escapetransversely through the wall of the cup. This can be achieved by makingthe foil of such dimensions that approximately 1 1/2 turns or more canbe wound around the stem. The foil may be clamped to the end of the stemby an embracing coil of wire, which also should be of refractory metalsuch as tungsten.

Embodiments of the invention will now be described in more detail by wayof example, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a thermionic electrode,

FIG. 2 is a side elevation. partly in section of the electrode shown inFIG. 1, and FIG. 3 is a perspective view, partly cut away of anotherform of thermionic cathode.

A thermionic cathode for use in a discharge lamp is shown in FIG. 1 and2 of the drawings and has a refractory metal supporting rod orcylindrical stem 10, for example in the form of a wire, the refractorymetal being tungsten. A tubular member 11, likewise of tungsten, issecured to the stem 10 adjacent the end 12 thereof and forms a cup 13.The tubular member can be regarded as a sleeve fitted to the end of thestem so as to project beyond the said end. As shown, the member 11 isformed by a refractory metal sheet or foil, of tungsten, which iswrapped around the stem. The width of the foil is greater than thecircumference of the stem 10. Thus there is an overlapping portion 14 ofthe foil which ensures that when the foil is wound or wrapped around thestem 10 it also wraps around upon itself. The width of the foil is suchthat, it can be wrapped approximately 1% times around the stem 10. As anexample of the dimensions involved, the tungsten stem may be 0.050 inchdiameter and the foil may be 0.001 inch in thickness and of the order of0.25 inch in width. The foil is secured or clamped to the end of thestem 10 by a tight encircling coil of wire 15. The wire 15 is a lengthof tungsten wire.

The cup 13 contains an electron-emitter composition consisting of W,9.5% Th and 0.5% Zr. After introducing the composition into the cup, forexample by pressing the dry, powdered composition therein, the assemblyis vacuum fired or annealed at a temperature in excess of 1,200C. Aprecaution to be noted is that if a different composition is utilized,namely one containing lanthanum or cerium, then care will be necessaryto exclude air whilst preparing the composition and before vacuumannealing. The reason for this precaution is that lanthanum and ceriumare spontaneously inflammable when finely powdered.

The composition need not completely fill the cup 13. It could in fact bea surface coating on the inner surface of the foil, and such a coatingcould be applied to the foil before winding or wrapping it around thestem 10. The composition could be applied to the foil by one of themethods described in the above-mentioned references and then sinteredthereon by vacuum annealing at a temperature in excess of 1,200 C. Stepsmust be taken to ensure that the overlapping portion 14 of the foil isfree from the surface coating.

The embodiment illustrated in FIG. 3 likewise has a refractory tubularmember 21 forming a sleeve around the stem 20. The member 21 projectsaway from the end 22 of the stem 20 to form a cup for theelectronemitter composition. The tubular member 21 in this case is inthe form of a helically-wound wire whose turns contact one another. Atleast those turns defining the cup are fuzed together. By so fuzing theturns, a tubular member 21 having a continuous wall is produced. Theturns may be fuzed together by arc welding or preferably byelectron-beam welding. The tubular member 21 can be secured tothe stem20 by welding, or simply by being a tight fit thereon.

The type of discharge lamp with which the cathode structures describedabove are particularly suited is the high pressure sodium or mercurylamp. Unlike conventional cathodes for discharge lamps which embodyalkaline earth oxides as emitters, the present cathode structures aresubstantially insensitive to atmospheric moisture andoxidation. Theconventional cathodes are very sensitive to becoming poisoned uponexposure to the atmosphere and are rendered ineffective thereby. Theyrequire most carefully controlled processing and handling than thepresent cathode structure. Furthermore, the cathodes described aboveenables efficient electron emission to be obtained at lower temperaturesthan is possible with cathodes consisting of plain metal emitters. Forexample, a pure tungsten cathode wire has to be heated to temperaturesof the order of l,OC higher than the present cathode for adequateemission to be obtained.

The cathode structures described, in common with shielded types ofcathode, possesses a further advantage over conventional open coilstructures and that is that blackening of the envelope of the dischargelamp is substantially eliminated. Blackening is prevented because thecontinuous wall of the cup 13 does not allow evaporated constituents ofthe electron-emitter composition to pass transversely out of the cup andreach the envelope of the discharge lamp.

It will be appreciated that an electrodes constructed as described abovecan be manufactured with ease. A

' stem; a refractory thermionic electrode would be extremely difficultto manufacture accurately by boring a hole in a refractory metal wire orstem because of the small overall dimensionsnormally required for suchelectrodes.

We claim:

- l. A discharge lamp having an improved thermionic cathode, saidcathode comprising: a refractory metal metal tubular member secured onone end of said stem, said tubular member extending beyond the end ofsaid stem to form a continuous walled cup; an electron-emittercomposition contained within said cup; the extensional direction of saidcup being substantially longitudinal with said discharge lamp, so thatevaporated constituents of said electronemitter composition do notdeposit on the envelope of said discharge lamp during normal lampoperation; said electron-emitting composition containing:

i. a refractory metal selected from the group consisting of tungsten,molybdenum and tantalum;

ii. a second metal selected from the lanthanide series of elements andthorium in Group Illa of the periodic classification, and

iii. a third metal that is at least partly soluble in the second metalto form an alloy therewith, said alloy being capable of wetting saidrefractory metal in said composition.

2. The combination according to claim 1, wherein said tubular member isa helically-wound wire-coil,

t ms of said coil contactin one another a d at 1 ast tliose turnsthereof which xtend beyond t he en of said stem to define said cup beingwelded together thereby forming a continuous wall.

3. The combination according to claim 1, wherein a foil wrapped aroundthe end of said stem forms said tubular member.

1. A discharge lamp having an improved thermionic cathode, said cathodecomprising: a refractory metal stem; a refractory metal tubular membersecured on one end of said stem, said tubular member extending beyondthe end of said stem to form a continuous walled cup; anelectron-emitter composition contained within said cup; the extensionaldirection of said cup being substantially longitudinal with saiddischarge lamp, so that evaporated constituents of said electron-emittercomposition do not deposit on the envelope of said discharge lamp duringnormal lamp operation; said electron-emitting composition containing: i.a refractory metal selected from the group consisting of tungsten,molybdenum and tantalum; ii. a second metal selected from the lanthanideseries of elements and thorium in Group IIIa of the periodicclassification, and iii. a third metal that is at least partly solublein the second metal to form an alloy therewith, said alloy being capableof wetting said refractory metal in said composition.
 2. The combinationaccording to claim 1, wherein said tubular member is a helically-woundwire-coil, turns of said coil contacting one another and at least thoseturns thereof which extend beyond the end of said stem to define saidcup being welded together thereby forming a continuous wall.
 3. Thecombination according to claim 1, wherein a foil wrapped around the endof said stem forms said tubular member.